A vehicle moving through the atmosphere is subjected to pressure forces distributed against the outside wall of the vehicle in a manner that depends on the shape of the vehicle and on its speed of travel. When the vehicle passes close to an obstacle (a tunnel mouth, a bridge, various objects in the vicinity of the track, . . .) or when it crosses another vehicle, the field of aerodynamic speeds entrained with the vehicle is deformed. As a result there are sudden changes in pressure on the outside walls of the vehicle. This effect becomes more marked with increasing vehicle speed, since the pressures involved are approximately proportional to the square of the speed.
For a vehicle in which inside air is renewed by air-conditioning or ventilation taken from outside air, the pressure inside the vehicle is close to the pressure outside the vehicle. Changes in outside pressure are transmitted almost instantaneously to the inside of the vehicle given the sectional dimensions of the ventilation circuits. It is observed that the time required for pressure changes to be transmitted to the inside of air-conditioned or ventilated vehicles is thus small, specifically because of the characteristics of the ventilation circuits. This rapid phenomenon does not occur when the inside of the vehicle is pressurized (e.g. in aircraft).
By way of example, when a high speed train enters a tunnel or a cutting, rapid changes in pressure occur on the outside walls of the coaches (or "cars"). These sudden changes propagate into the confined space constituted by the tunnel or the cutting and are known by the term "pressure waves". In addition, such pressure waves which propagate in the outside confined space are reflected from the ends thereof, thereby creating reflected pressure waves that are as large as the direct waves. When the coaches possess a fair degree of sealing, these changes travel quickly to the insides of the coaches via their ventilation and air-conditioning circuits, causing considerable discomfort to passengers. It is thus common to be subjected to rates of change in pressure that may be as great as 2,000 Pascals in less than 1 second.
To remedy this pressure wave problem, certain high speed trains use "steep front" fans placed in the ventilation circuit. These fans form, to some extent, an obstacle to pressure waves, but they have the drawback of being expensive and noisy. They constitute extra equipment of non-negligible weight and bulk in spite of the fact that major efforts are made to lighten coaches as much as possible. They consume large amounts of power. In addition, it is necessary to have two per coach.